The move comes more than a week after the close of sign-ups for insurance coverage
on the law.

After a rocky start that Sebelius was blamed for, the administration rebounded
strongly to exceed expectations by enrolling 7.1 million people by the March 31
deadline.

"Under Kathleen's leadership, her team at HHS turned the corner, got it fixed, got
the job done," Obama said in a Rose Garden ceremony Friday morning. "And the final
score speaks for itself."

The nomination of Burwell, who was unanimously confirmed by the Senate for her
current post last year, appeared aimed at avoiding an election-year confirmation
fight.

"Last time, she was confirmed unanimously," Obama said. "I'm assuming not much has
changed since that time."

Obama remained publicly supportive of Sebelius throughout the rough rollout,
deflecting Republican calls for her resignation. But she was conspicuously not
standing by his side last week when he heralded the sign-up surge during an event
in the White House Rose Garden.

A White House official said the 65-year-old
Sebelius approached Obama last month about stepping down, telling him that the
sign-up deadline was a good opportunity for a transition and suggesting he would
be better served by someone who was less of a political target.

A
spokeswoman for Sen. Pat Roberts, a Republican from Sebelius' home state of
Kansas, called the resignation "a prudent decision" given what she called the
total failure of Obamacare implementation.

Sebelius dropped no hints about
her resignation Thursday when she testified at a budget hearing. Instead, she
received congratulations from Democratic senators on the sign-up surge.

A
popular former governor of Kansas, Sebelius has been one of Obama's longest-
serving Cabinet officials and his only HHS secretary. She was instrumental in
shepherding the health care law through Congress in 2010 and implementing its
initial components, including a popular provision that allows young people to stay
on their parents' insurance plans until age 26.

But Sebelius' relationship
with the White House frayed during the fall rollout of the insurance exchanges
that are at the center of the sweeping overhaul. The president and his top
advisers appeared caught off guard by the extent of the website woes, with
warnings from those working on the technology never making it to the West Wing.

After technical problems crippled online sign-ups after the Oct. 1 launch,
the White House sent management expert and longtime Obama adviser Jeffrey Zients
to oversee a rescue operation that turned things around by the end of November.
After taking helm of the project, Zients said management issues were partly to
blame but did not point the finger at any individuals.

Sebelius took
personal responsibility for the chaotic launch of the website and asked the HHS
inspector general to conduct an investigation. That report is not expected for
months.

House Minority Leader Nancy Pelosi, a staunch supporter of the
health care law, praised Sebelius as a "forceful, effective and essential"
secretary.

"Secretary Sebelius was a leader in the long effort to make
history for our country with passage of the Affordable Care Act," the California
Democrat said in a statement.

In nominating the 48-year-old Burwell, Obama
is tapping a Washington veteran with a low-profile and the respect of some
Republicans on Capitol Hill. Though she only joined the Obama administration last
year, Burwell held several White House and Treasury posts during President Bill
Clinton's administration.

Between her stints in the executive branch,
Burwell served as president of Wal-Mart's charitable arm and head of the global
development program at the Bill and Melinda Gates Foundation.

If confirmed,
Burwell will have to contend with huge challenges related to the continued
implementation of the health overhaul, as well as the divisive politics
surrounding the law that show no sign of abating.

On the practical side, the
administration has to improve customer service for millions of Americans trying to
navigate the new system. There's also a concern that premiums may rise for 2015,
since many younger, healthier people appear to have sat out open enrollment
season.

On the political front, congressional Republicans remain implacably
opposed to Obamacare, even as several GOP governors have accepted the law's
expansion of safety-net coverage under Medicaid. GOP opposition means Republicans
can be expected to continue to deny additional funds for implementation.

Senate Minority Leader Mitch McConnell., R-Ky., welcomed Sebelius' resignation
but appeared to indicate an openness to a dialogue with Burwell, the new HHS
nominee -- even as he declared that "Obamacare has to go."

"I hope this is
the start of a candid conversation about Obamacare's shortcomings and the need to
protect Medicare," McConnell said.

___

Associated Press writer Ricardo
Alonso-Zaldivar and Erica Werner in Washington, and John Hanna in Topeka, Kan.,
contributed to this report.

___

Follow Julie Pace at
http://twitter.com/jpaceDC

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